In this work, four runners for a gravitational vortex turbine (GVT) were initially analyzed numerically with the aim of selecting the one with the best efficiency (η). The selected rotor was optimized using the response surface methodology (RSM). Four design factors were considered: the number of blades (M), the twist angle (λ), the relationships between the runner upper (D) and lower (d) diameters, and the discharge chamber diameter (D_c); i.e., D/D_c and d/D_c, respectively. For the numerical analysis, a three-dimensional (3D) computational domain in ANSYS Fluent software with the K-ε RNG turbulence model and the six degrees of freedom (6-DoF) user defined function (UDF) method was utilized for the unsteady flow simulations. The η versus (vs.) the angular velocity (ω) curve was monitored during the CCD for all the treatments tested. The highest η was 0.522 under optimal design conditions; i.e., for M, λ, D/D_c and d/D_c equal to 6, 55°, 0.5 and 0.23, respectively. The optimal runner was built using a 3D printing and was experimentally tested utilizing a hydraulic bench. The experimental and numerical η vs. ω curves were compared. A difference of 5.1% between the maximum values of η was found.